The wave of electricity market restructuring both within the United States and abroad has brought the issue of horizontal market power to the forefront of energy policy. Traditionally, estimation and prediction of market power has relied heavily on concentration measures . In this paper, we discuss the weaknesses of concentration measures as a viable measure of market power in the electricity industry, and we propose an alternative method based on market simulations that take advantage of existing plant level data. We discuss results from previous studies the authors have performed , and present new results that allow for the detection of threshold demand levels where market power is likely to be a problem. In addition, we analyze the impact of that recent divestitures in the California electricity market will have on estimated market power. We close with a discussion of the policy implications of the result